Device and method for detecting the spatial position of the optical axis of an eye and for centering a reference system relation to the optical axis

ABSTRACT

A method and a device for detecting the spatial position of the optical axis of the eye of a human or animal subject and for centering a reference system in relation to the optical axis are described, having at least one light source emitting a parallel light beam bundle, a positioning region for the subject provided opposite the light source, means for relative position orientation of the parallel light beam bundle in relation to the eye of the subject, and at least one detector unit for detecting reflection events caused in and on the eye by the parallel light beam bundle. The present invention generates control signals on the basis of the scattering and reflection events detected by the detector unit, by which the means for relative position orientation are activated, the control signals being generated in such a way that at least one reflection event and at least one scattered light event are to be brought into congruency in relation to the propagation direction of the parallel light beam bundle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device and a method for detecting thespatial position of the optical axis of the eye of a human or animalsubject and for centering a reference system in relation to the opticalaxis, having at least one light source, which emits a parallel lightbeam bundle, a positioning region for the subject provided opposite thelight source, means for relative position orientation of the parallellight beam bundle in relation to the eye of the subject, and at leastone detector unit for detecting reflection events caused in and on theeye by the parallel light beam bundle.

2. Description of the Prior Art

Devices of the species described above are predominantly used in thefield of ophthalmology, particularly for correcting the defective sightof an eye using a laser treatment system, particularly an excimer laser,to provide a targeted material removal on or inside the cornea, toachieve a desired change of the corneal curvature and a correction ofthe optical action of the cornea connected thereto.

The photorefractive correction of defective sight has been a recognizedand very effective method for correcting vision errors for years. In themeantime, all types of defective sight such as nearsightedness (myopia),farsightedness (hyperopia), and corneal curvatures (astigmatism) havebeen treated very successfully using appropriately equipped lasersystems. A requirement for successful treatment requires exactpositioning of the treatment laser beam in relation to the cornea to betreated.

With increasing precision of the excimer laser systems and the precisionof the diagnostic methods available, exact positioning of the treatmentlaser beam at the location of the cornea gains ever greatersignificance. A few years ago, the photorefractive correction ofdefective sight was modified by the simple use of the spectacleprescription as starting information in such a way that the spatiallyresolved aberrations of the entire system of the eye were measured usingwavefront technology and the topographic properties of the cornea, and acorresponding correction guideline for the treatment laser beam wasprepared.

It is a direct consequence that with the significant increase ofdetailed information available to the photorefractive correction,precise positioning of the eye to be treated and/or the treatment at thelocation of the cornea gains significance.

Treatment laser systems of this type are generally supported by eyetracker systems, which are based on greatly varying method technologies.With the aid of systems of this type, position changes of the eye in themagnitude of less than 100 μm may be recognized.

An essential requirement for successful correction of defective sight isthus precise orientation and positioning of the treatment laser beam inrelation to the eye to be treated and its optical axis, and/or theoptical axes of its refractive partial faces. The practice typical untilnow for adjusting a treatment laser beam in relation to the eye wasperformed up to this point in relation to the pupil center or purelysubjectively by the treating physician, who oriented himself either onunchanging eye features or manually adjusted the treatment laser beam asa function of light reflections occurring on or in the eye. Alllaser-supported refractive surgery methods known up to this point foroptimized correction of defective sight in the eye therefore lack anobjectively repeatable adjustment of the treatment laser beam inrelation to the eye to be treated and its optical axis, and/or theoptical axes of its refractive partial faces.

SUMMARY OF THE INVENTION

The invention is a device and a method, which the treating physicianuses when performing laser-supported photorefractive correction on aneye, and which may automatically adjust the treatment laser beamrequired for this purpose under exclusively objective frameworkconditions in relation to the eye. It is thus possible to implement anobjective and exactly reproducible fixing of the optimum position of thetreatment laser beam to perform the laser treatment on the cornea, whichis independent of the operating precision of the particular operator.

The device according to the present invention allows the physician toorient the treatment laser beam on the basis of the individual opticalaxis of the eye to be treated and/or its refractive partial face to betreated, which may be determined automatically by the device accordingto the present invention.

For this purpose, the device for detecting the spatial position of theoptical axis of the eye of the human or animal subject and for centeringa reference system, preferably a treatment laser beam, in relation tothe optical axis of the eye has the following components. It is notedhere that the device is also usable on animals, but the furtherembodiments are restricted, without restricting the invention,exclusively to eye correction and/or determining the optical axis in thehuman eye. The components are:

-   -   at least one light source, which emits a parallel light beam        bundle, for which a laser emitting in the visible or        near-infrared spectral range is preferably suitable, whose light        wavelength is not to unfold any therapeutic effect on the eye;    -   a positioning region for the subject, provided opposite the        light source, so that the location of the eye to be treated        assumes a largely spatially defined position. A head support        adaptable to the head contour is preferably used for this        purpose, in which the head of the subject may be laid spatially        fixed;    -   means for relative position orientation of the parallel light        beam bundle in relation to the eye of the subject. In the        simplest case, the means are a substrate implemented as an x/y        positioning table, which is movable in a controlled way in at        least one plane, which intersects the beam direction of the        parallel light beam bundle perpendicularly, the positioning        table may especially preferably also be moved in the beam        direction, that is, in the z direction; and    -   at least one detector unit for detecting reflection and        scattered light events caused in and on the eye by the parallel        light beam bundle. In principle, any type of light-sensitive        detector system is suitable and preferably an imaging camera        system, such as a video camera, is suitable for the detector        unit.

The device is distinguished according to the present invention in thatan analysis unit is provided, which generates control signals on thebasis of the reflection and scattered light events detected by thedetector unit, by which the means for relative location orientation areactivated, the control signals being generated in such a way that atleast one reflection event and at least one scattered light event are tobe brought into congruency in relation to the propagation direction ofthe parallel light beam bundle.

The device according to the present invention, which makes it possibleto automatically find at least one reflection event occurring in or onthe eye and one scattered light event, determines the visual and/oroptical axis of the eye to be treated and/or one or more of itsrefractive partial faces by bringing a reflection event occurring on orin the eye and a scattered light event into congruency. Subsequently,the treatment laser beam required for performing the photorefractivecorrection is oriented knowing the spatial position of the optical axis.

The eye of the subject is automatically moved in relation to theparallel light beam bundle to find and/or detect the optical axis andfor a readjustment between the treatment laser beam and the optical axiswhich is required while performing a photorefractive correction, so thatexact orientation between eye and light source and finally the referencesystem to be viewed as the treatment laser beam is ensured during theentire engagement. For this purpose, the control signals, which are usedfor activating a x-y adjustment table, on which the subject rests,corresponding to the reflection and/or scattered light events detectedby the detector unit, are generated using computer-supported imageprocessing. An adjustment table which is additionally movable in height,that is, in the z direction and/or along the beam direction of theparallel light beam bundle, is preferably suitable.

In principle, reflection and/or scattered light events result in or onthe eye due to the parallel light beam bundle at interfaces inside theeye, at which two material layers having different indices of refractionmeet.

Light beams are thus refracted and reflected on various surfaces of theeye. A first reflection of a light beam incident on the eye surfaceoccurs at the surface of the cornea itself and is typically alsoreferred to as a corneal reflection. The corneal reflection is alsoreferred to in the literature as the first Purkinje image. The second,third, and forth Purkinje images arise at the interfaces ofcornea/aqueous humor, aqueous humor/lens, and lens/vitreous humor of theeye, respectively.

In addition, when a parallel light beam bundle shines through the corneaof the eye as the first optical component of the eye, the cornea exertsan optical effect on the beam bundle comparable to that of a converginglens, which typically has a radius of curvature in the magnitude ofapproximately 7.4 mm. To find the optical axis of the eye, at least twolight points lying on or in the eye, which are caused by the beambundle, must be recognized and brought into congruency.

If the eye of the patient is illuminated using a parallel light beambundle, the patient is to fixate on the punctual light source, fromwhich the parallel light beam bundle originates, appearing optically tothe patient to be treated as a punctual light source. Upon observationof the eye, the following light appearances and/or light points may beestablished as a result of reflection and scattering effects on and inthe eye:

-   -   the fixation light source appearing as a scattered light event,    -   the fovea centralis of the retina, because of the fixation by        the patient, and    -   the first Purkinje image of the fixed light source, which        appears as a reflection event on the cornea surface.

If a fixation light source having a parallel light beam bundle is used,such as an LED, the passage point of the parallel light beam bundlethrough the cornea, which may be recognized from a weak scattered lightreflection on the cornea surface, may be moved over a comparativelybroad area of the cornea without the patient looking after the fixationlight source, whose location is changing, since the patient thinks thepatient sees the light source in infinity because of its parallel beamproperty and does not recognize the position displacement of the lightsource, which is actually located in the finite. If the passage point ofthe parallel light beam deviates further from the location of theoptical axis of the eye, that is, if the passage point is eccentric, thescattered light reflection or the scattered light event of the fixationlight source appears less bright the more eccentric the passage point.

To now find the penetration point of the parallel light beam through thecornea surface which lies exactly on the optical axis of the eye, thescattered light event on the cornea surface and, in addition, the firstPurkinje image arising as a reflection event in the fixation lightsource—under the condition of central fixation by the patient—must bealigned, that is, lie along a shared spatial axis. This is precisely thecase when the first Purkinje image coaxial with the fixation light beamcauses a strong, that is, especially bright reflection phenomenon on theeye for the observer, that is, for the treating physician. In thisconstellation, the parallel light beam incident on the cornea surface atthe location of the first Purkinje image is reflected back into itselfand is additionally superimposed with the scattered light event of thefixation light source. It is obvious that a light point of maximum lightintensity unmistakably results for the treating physician, whichrepresents a unique navigation aid for determining the spatial positionof the optical axis of the eye of the patient to be examined andtreated.

This may be applied accordingly for other refractive partial faces ofthe eye to determine their position.

The configuration thus described offers a practically locationally fixedindication of the location at which the optical axis of the cornea“penetrates” its surface under the fixation condition on the basis ofthe properties discussed above. This penetration point, however, is themost suitable center for a change of curvature which alters refractivepower, for example, with the aid of a laser.

On the basis of these properties, the detector unit of the deviceaccording to the present invention has at least one optical imagingsystem, by which the particular scattered light and reflection eventsoccurring in the area of the air/cornea interface may be imaged sharply.

Through the optical detection of the first Purkinje image (reflectionevent) and the scattered light event on the detector unit, which ispreferably implemented as a video camera, whose field of vision isdirected at least on the pupil area of the eye and which, in addition,provides a location-resolving image plane for detecting the position ofthe reflection and scattered light events which may be imaged in theimage plane of the camera, using a computer-supported image analysisunit, the distance and the relative positions between the cornealreflection and the scattered light event are ascertained. Moreover, thecorneal reflection and the scattered light event are brought intocongruency by targeted position change of the eye. A trajectory iscalculated on the basis of the position and distance informationobtained during the image analysis, along which the eye is to be movedto bring both light events into congruency and finally to be stopped inthis position, at least until a photorefractive correction has beencompleted.

As already noted, this is performed by actively monitored regulatedrelative motion of the subject in relation to the parallel light beambundle.

If the optical axis of the eye has been detected in the above way andcorrespondingly adjusted in relation to the parallel light beam bundle,a treatment laser beam may be coupled along the parallel light beambundle to perform the photorefractive correction, which performs desiredmanipulations on the eye by targeted ablation or coagulation of tissueareas in or on the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained for exemplary purposes in thefollowing with restriction of the general idea of the present inventionon the basis of an exemplary embodiment with reference to the drawing.

FIG. 1 shows an illustration of reflection events occurring on the eye;

FIG. 2 shows an illumination situation having a centered optical axis;

FIG. 3 shows a schematic illustration of the device for automaticallyfinding the optical axis through an eye;

FIG. 4 shows beam geometry for the occurrence of a virtual image withinan eye; and

FIG. 5 shows an illustration of the location changeability of thevirtual image.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic partial cross-section through a human eye 6,which has a cornea 1, aqueous humor 2, a lens 3, and the vitreous humorand/or eye inner chamber 4. If a light beam L is incident on the surfaceof the cornea 1, a part of the light beam is reflected at the interfaceair/cornea surface. The reflection event P1 in this regard is referredto as the Purkinje image P1 or as the corneal reflection. Similarreflection events P2-P4 occur at the interfaces of cornea 1/aqueoushumor 2, aqueous humor 2/lens 3, and lens 3/vitreous humor 4.

Moreover, the reflection event P1 at the cornea surface and a scatteredlight event PS occurring in the course of the optical scattering at thecornea surface are used for ascertaining the visual axis A of the entiresystem, as described at the beginning.

At this point, an alternative possible occurrence of the scattered lightevent PS on the eye is noted only for reasons of completeness, which mayalso be assumed to be conceivable from the current understanding of theoptical properties of the eye and changes nothing in the aboveconsiderations. For this purpose, reference is made to FIG. 4, whichshows the beam geometry of an optical image of an object O on the cornea1, which acts as a hollow mirror, and has a hollow mirror focal point B.The scattered light event PS occurring in the interior of the eye may becomprehended as an image of the light source by the optical hollowmirror effect and is used for exactly determining the optical axis A ofthe eye. A punctual light source 9 is used for this purpose as afixation target (see FIG. 5), from which a parallel light beam bundle Loriginates and which is directed toward the eye 6 of the patient. In thecase of the position x1 shown in FIG. 5, the scattered light event PS islocated outside the optical axis A. The position x2 also lies outsidethe optical axis A, but x2 is to illustrate that the position x2 isassumed by the scattered light event PS when the eye has moved orshifted, that is, the position of the scattered light event PS has itsposition dependent on the eye of the patient in relation to the parallellight beam bundle L.

The penetration point DP of the optical axis A through the cornea isfound exclusively in the case in which the scattered light event PS andthe corneal reflection 1 are congruent along the parallel light beambundle L. See position x3 of the scattered light event PS. In this case,parts of the parallel light beam bundle L are reflected back at thecornea surface into itself and the scattered light event PS is congruentwith this reflection. This constellation is distinguished by a markedbrightening of the reflection to be observed on the eye.

The goal is thus to provide an illumination situation in which theparallel light beam bundle L incident on the eye 6 is oriented coaxiallyto the optical axis A. This is the case when the reflection event P1 andthe scattered light event PS, as shown in the illustration of FIG. 2,are brought into congruency with one another. Two circular observationfields 5 are shown for better illustration in FIG. 2, which show thespatial positions of P1 and PS for an illumination of the eye using aparallel light beam path. The left observation field represents theimage in which the optical axis A of the eye is maladjusted in relationto the parallel light beam path L. In this case, the spatial position ofthe reflection event on the cornea surface P1 deviates from the positionof the scattered light event PS, which is already located along theoptical axis A. To find the optical axis A which already runs throughthe scattered light event PS, the corneal reflection P1 in the beamdirection of the parallel light beam bundle L illuminating the eye is tobe brought into congruency with the scattered light event PS. This isshown in the right circular image illustration of FIG. 2. If both lightevents P1 and PS are congruent, their connecting axis is coincident withthe visual or optical axis A of the eye, by which the spatial positionof the eye is detected and fixed.

The device schematically illustrated in FIG. 3 is capable ofautomatically ascertaining the optical axis of the eye 6 and, inaddition, tracking and/or fixing the detected optical axis of the eye 6accordingly in relation to an optical reference system, for example, atreatment laser beam. For this purpose, the eye 6 to be treated of asubject (not shown further), who lies on a patient rest 7, which ismovable in the x-y plane around at least two spatial axes via actuators8 driven by motors, is positioned. A light source 9 emits a parallellight beam bundle L, which is directed into the pupil area of the eye 6,to generate the reflection and scattered light events on and in the eye6 described above. The reflection and scattered light events occurringon or in the eye are imaged via an imaging object (not shown further) ina detector unit 10 via a deflection unit 14, in which the viewing fields5 shown in FIG. 2 result. The detector unit 10 is preferably implementedas a video camera and is used for location-resolving position detectionof the reflection and scattered light events imaged in the image planeof the video camera in regard to the Purkinje image P1 and the scatteredlight event PV, as explained above. Preferably, a separate imagingsystem and/or camera system may also be used in each case to detect thetwo light events, in order to sharply image both light events, which arepossibly spaced apart from one another in depth.

The light events P1 and PS are to be brought in congruency in relationto the beam direction of the parallel light beam bundle L with the aidof a computer-supported graphic image analysis unit 11. For thispurpose, a trajectory is ascertained, using which the light events P1and PS, which initially are spatially separated, may be brought intocongruency with one another. For this purpose, the analysis unit 11generates control signals, which are transmitted to the adjustment unit8, by which the patient rest is brought into a corresponding position.The procedure of bringing the two light events P1 and PS arising insidethe eye 6 into congruency is performed completely automatically. As soonas the light events P1 and PS are congruent in the beam direction of theparallel light beam bundle L, their connecting axis defines the visualand/or optical axis of the eye 6. Further optical manipulations may beperformed on the eye in precisely this configuration. For example,coupling a treatment laser beam 12 into the beam path of the parallellight beam bundle L with the aid of a semitransparent deflection mirror13 is suitable. The treatment laser beam 12 is preferably used forperforming photorefractive correction measures in or on the eye. If theposition of the optical axis changes during the treatment in relation tothe parallel light beam bundle and also to the treatment laser beam, amaladjustment is detected by the detector unit on the basis of thechanging positions of the particular light events P1 and PS andcorresponding correction control signals are generated under real-timeconditions for readjustment of the eye.

The device according to the present invention schematically shown inFIG. 3 is thus used for finding the optical axis of an eye completelyautomatically and for its spatial fixation in the course of a regulatedadjustment and/or tracking of the eye in relation to an opticalreference system to perform photorefractive correction measures on theeye in particular.

LIST OF REFERENCE NUMERALS

-   1 cornea-   2 aqueous humor-   3 lens-   4 vitreous humor-   5 field of vision-   6 eye-   7 positioning region, patient rest-   8 adjustment unit-   9 light source-   10 detector unit-   11 analysis unit-   12 treatment laser beam-   13 deflection mirror-   14 deflection mirror-   L parallel light beam bundle-   DP penetration point-   A optical axis-   B focal point of the cornea-   PS scattered light event

1. A device for detecting a spatial position of an optical axis of aneye of a human or animal subject and for centering a reference system inrelation to the optical axis, including at least one light sourceemitting a parallel light beam bundle, a positioning region for thesubject provided opposite the light source, means for the relativeposition orientation of the parallel light beam bundle in relation tothe eye of the subject, and at least one detector unit for detectingreflection events caused in and on the eye by the parallel light beambundle, wherein an analysis unit generates control signals on a basis ofscattering and reflection events detected by the at least one detectorunit, by which the means for relative position orientation areactivated, the control signals being generated so that at least onereflection event and at least one scattered light event are to bebrought into congruency in relation to a propagation direction of theparallel light beam bundle.
 2. The device according to claim 1, wherein:the eye has at least four optically active interfaces in the irradiationdirection of the parallel light beam comprising air/cornea,cornea/aqueous humor, aqueous humor/lens, and lens/vitreous humor; andthe detector unit has at least one optical imaging system, by which areflection event occurring in an area of the air/cornea interface, (thefirst Purkinje image), and a scattered light event occurring on the eyemay be imaged sharply.
 3. The device according to claim 2, wherein: thedetector unit provides at least one camera, including at least one fieldof vision directed to the pupil area of the eye and a position-resolvingimage plane for position detection of the reflection and/or scatteredlight events which may be imaged in the image plane of the video camera.4. The device according to claim 2, wherein: the means for relativeposition orientation of the parallel light beam bundle in relation tothe eye of the patient includes at least one adjustment unit, by whichthe positioning region provided for the subject is movable in relationto the stationary light source in at least one plane.
 5. The deviceaccording to one of claim 2, wherein: the analysis unit provides acomputer-supported graphic image analysis unit, which automaticallydetects at least the first Purkinje image and the scattered light eventand calculates a trajectory to bring and keep two light events incongruency by relative position change between the light source and eyeof the subject.
 6. The device according to claim 2, comprising: afurther light source, whose light beam is selectively coupled into theparallel light beam directed onto the eye via at least one opticaldeflection element.
 7. The device according to claim 1, wherein: thedetector unit provides at least one camera, including at least one fieldof vision directed to the pupil area of the eye and a position-resolvingimage plane for position detection of the reflection and/or scatteredlight events which may be imaged in the image plane of the video camera.8. The device according to claim 7, wherein: the means for relativeposition orientation of the parallel light beam bundle in relation tothe eye of the patient includes at least one adjustment unit, by whichthe positioning region provided for the subject is movable in relationto the stationary light source in at least one plane.
 9. The deviceaccording to one of claim 7, wherein: the analysis unit provides acomputer-supported graphic image analysis unit, which automaticallydetects at least the first Purkinje image and the scattered light eventand calculates a trajectory to bring and keep two light events incongruency by relative position change between the light source and eyeof the subject.
 10. The device according to claim 1, wherein: the meansfor relative position orientation of the parallel light beam bundle inrelation to the eye of the patient includes at least one adjustmentunit, by which the positioning region provided for the subject ismovable in relation to the stationary light source in at least oneplane.
 11. The device according to one of claim 10, wherein: theanalysis unit provides a computer-supported graphic image analysis unit,which automatically detects at least the first Purkinje image and thescattered light event and calculates a trajectory to bring and keep twolight events in congruency by relative position change between the lightsource and eye of the subject.
 12. The device according to one of claim1, wherein: the analysis unit provides a computer-supported graphicimage analysis unit, which automatically detects at least the firstPurkinje image and the scattered light event and calculates a trajectoryto bring and keep two light events in congruency by relative positionchange between the light source and eye of the subject.
 13. The deviceaccording to claim 1, comprising: a further light source, whose lightbeam is selectively coupled into the parallel light beam directed ontothe eye via at least one optical deflection element.
 14. The deviceaccording to claim 13, wherein: the further light source is a treatmentlaser for targeted ablation and/or coagulation of tissue areas on or inthe eye.
 15. The device according to claim 13, wherein: the light beamof the further light source and/or the parallel light beam bundlerepresents the reference system.
 16. In a device detecting a spatialposition of an optical axis of an eye of a human or animal subject andfor centering a reference system in relation to the optical axis,including at least one light source emitting a parallel light beambundle, a positioning region for the subject disposed opposite the lightsource, means for the relative position orientation of the parallellight beam bundle in relation to the eye of the subject, and at leastone detector unit for detecting reflection events caused in and on theeye by the parallel light beam bundle, wherein an analysis unitgenerates control signals on a basis of scattering and reflection eventsdetected by the at least one detector unit, by which the means forrelative position orientation are activated, the control signals beinggenerated so that at least one reflection event and at least onescattered light event are to be brought into congruency in relation to apropagation direction of the parallel light beam bundle, a methodcomprising: detecting a penetration point of the optical axis of the eyethrough the cornea; and automatically tracking an optical exposition inrelation to the penetration point of the optical axis to the cornea. 17.The method according to claim 16, wherein: the optical exposition is atherapeutic or diagnostic energy beam, or an optical axis of an imagingoptic.
 18. A method for detecting a spatial position of an optical axisof an eye of a human or animal subject and for centering a referencesystem in relation to the optical axis, in which the eye is illuminatedusing at least one parallel light beam bundle, the eye of the subjectand/or the parallel light beam bundle are positioned in relation to oneanother so their positions may be changed and reflection events causedby the parallel light beam bundle on and/or inside the eye are detectedusing a detector unit, comprising: detecting a reflection event causedby the parallel light beam bundle on the cornea surface and a scatteredlight event caused on the eye using the detector unit; and changing theeye of the subject in its position in relation to the parallel lightbeam bundle so that the reflection event on the cornea surface and thescattered light event are brought into congruency along the parallellight beam bundle and held there.
 19. The method according to claim 18,wherein: for axial coincidence between a reflection event occurring onthe cornea surface and a scattered light event, a reflection positionconnected to the reflection event is determined as a penetration pointof the optical axis of the eye through the cornea.
 20. The methodaccording to claim 19, wherein: a spatial position of the penetrationpoint is stored.